Deformation and fracture of a particle-reinforced aluminum alloy composite: Part I. Experiments

Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA) composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response. The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450 °C as-rolled condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged (PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)-based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part II. Modeling

In this article, the tensile and fracture properties of a discontinuously reinforced aluminum (DRA) alloy composite are modeled to determine the influence of constituent parameters on material behavior. Comparison of the elastic-modulus calculations to the experimental data suggest that the angular particles are more effective in load transfer than spherical particles, and that a unit cylinder geometry is a good representation of the particles under elastic conditions. This same geometry is used in the finite element-based elastic-plastic model of Bao et al., and reasonably good agreement is obtained between the experimental and predicted yield strengths. A fracture-mechanics model is proposed for predicting the elongation to failure. The model assumes the existence of particle cracks, and criticality is based on the strain required for matrix rupture between cracked particles. The damage criterion of Cockcroft and Latham is utilized, and model predictions are compared to data from different investigations. It is shown that the volume fraction of particles and the work-hardening coefficient of the matrix have a strong influence on the strain to failure. Fracture toughness modeling one again exposes the limitations of existing zero-degree crack-propagation models, such as that of Hahn and Rosenfield, which predict increased toughness with yield strength rather than a decrease, which is observed experimentally. A shear-failure model along a 45-deg direction is proposed for the higher-strength conditions, where concentrated slip bands were observed. The model exhibits the inverse toughness dependence on strength and better correlation to peak-aged (PA) data, but shows poorer agreement with underaged (UA) data. Thus, a transition from zero-degree propagation to 45-deg propagation with increasing strength is suggested. A simplified method for extracting particle stresses is illustrated and is used to estimate a Weibull modulus of 4.9 and a Weibull strength of 2450 MPa for the SiC particles of an average diameter of 10 μm. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Deformation and fracture of a particle-reinforced aluminum alloy composite: Part II. Modeling

In this article, the tensile and fracture properties of a discontinuously reinforced aluminum (DRA) alloy composite are modeled to determine the influence of constituent parameters on material behavior. Comparison of the elastic-modulus calculations to the experimental data suggest that the angular particles are more effective in load transfer than spherical particles, and that a unit cylinder geometry is a good representation of the particles under elastic conditions. This same geometry is used in the finite element-based elastic-plastic model of Bao et al., and reasonably good agreement is obtained between the experimental and predicted yield strengths. A fracture-mechanics model is proposed for predicting the elongation to failure. The model assumes the existence of particle cracks, and criticality is based on the strain required for matrix rupture between cracked particles. The damage criterion of Cockcroft and Latham is utilized, and model predictions are compared to data from different investigations. It is shown that the volume fraction of particles and the work-hardening coefficient of the matrix have a strong influence on the strain to failure. Fracture toughness modeling once again exposes the limitations of existing zero-degree crack-propagation models, such as that of Hahn and Rosenfield, which predict increased toughness with yield strength rather than a decrease, which is observed experimentally. A shear-failure model along a 45-deg direction is proposed for the higher-strength conditions, where concentrated slip bands were observed. The model exhibits the inverse toughness dependence on strength and better correlation to peak-aged (PA) data, but shows poorer agreement with underaged (UA) data. Thus, a transition from zero-degree propagation to 45-deg propagation with increasing strength is suggested. A simplified method for extracting particle stresses is illustrated and is used to estimate a Weibull modulus of 4.9 and a Weibull strength of 2450 MPa for the SiC particles of an average diameter of 10 μm. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Relationship between fracture toughness, fracture path, and microstructure of 7050 aluminum alloy: Part I. Quantitative characterization

The fracture toughness of Al-Zn-Mg-Cu-based 7XXX aluminum alloys decreases with an increase in the extent of recrystallization. In this contribution, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the 7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging condition. The fracture path is quantitatively correlated to fracture toughness, and the bulk microstructural attributes estimated via stereological analysis. In the companion article, these quantitative data are used to develop and verify a multiple-fracture micromechanism-based model that relates the fracture toughness to a number of microstructural parameters of the partially recrystallized alloy plate.     

Recrystallization and grain growth phenomena in a particle-reinforced aluminum composite

Recrystallization and grain growth in a 2219/TiC/15p composite were investigated as functions of the amount of deformation and deformation temperature. Both cold and hot deformed samples were annealed at the normal solution treatment temperature of 535 °C. It was shown that large recrystallized grain diameters, relative to the interparticle spacing, could be produced in a narrow range of deformation for samples cold-worked and those hot-worked below 450 °C. For cold-worked samples, between 4 to 6 pct deformation, the recrystallized grain diameters varied from 530 to 66 μm as the amount of deformation increased. Subsequent grain growth was not observed in these recrystallized materials and noncompact grain shapes were observed. For deformations greater than 15 pct, recrystallized grain diameters less than the interparticle spacing were observed and subsequent grain growth produced a pinned grain diameter of 27 μm. The pinned grain diameter agreed well with an empirical model based on three dimensional (3-D) Monte Carlo simulations of grain growth and particle pinning in a two-phase material. Tensile properties were determined as a function of grain size, and it was shown that grain size had a weak influence on yield strength. A maximum in the yield strength was observed at a grain size larger than the normal grain growth and particle-pinned diameter.     

Forging limits for an aluminum matrix composite: Part I. Experimental results

Forging limits in a discontinuously reinforced aluminum (DRA) matrix composite, 2014 Al/15 vol pct A1^-2O^-3, were determined by compressing samples of various cylindrical geometries under different conditions of temperature, strain rate, and lubrication and measuring the limit strains attained prior to incipient crack formation. In some cases, circumferential grids were machined on the sample surface to obtain the local fracture strain states. Crack formation was caused by the secondary tensile stresses; however, crack propagation was relatively slow and somewhat more severe at 300 °C than at 400 °C. The forging limit of the composite was found to be higher at 400 °C than at 300 °C and also higher at slower strain rates. The plane-strain forging limit of the composite at 300 °C and a strain rate of 0.5 s”1 was less than 0.05, while that of the matrix was higher than 0.5. It was found that the forging limits can be influenced by the depth of the circumferential grids and can be lower than those for the smooth surface samples. Formerly Graduate Student, the University of Michigan     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part I. Experiments

This article focuses on understanding the mechanical behavior of precipitation-hardened alloys by studying single and polycrystalline deformation behavior with various heat treatments. Aluminumcopper alloys are the focus in this work and their changing stress-strain behavior is demonstrated resulting from the different hardening mechanisms brought about by the various precipitates. Extensive transmission electron microscopy investigations facilitated the interpretation of the stress-strain behavior and the work hardening characteristics. The use of both single and polycrystals proved valuable in understanding the role of anisotropy due to crystal orientation vs precipitate-induced anisotropy. The experiments show that precipitation-induced anisotropy could offset the crystal orientation anisotropy depending on the orientation. This is clearly demonstrated with similar [111] and [123] behaviors under 190 °C and 260 °C aging temperatures. Experiments on pure aluminum crystals are also provided for comparison and understanding the crystal anisotropy in the absence of precipitates. Part I of this article will focus on experiments, and part II will describe the modeling of the effect of different metastable phases in the matrix acting as barriers to dislocation motion.     

Precipitate effects on the mechanical behavior of aluminum copper alloys: Part I. Experiments

This article focuses on understanding the mechanical behavior of precipitation-hardened alloys by studying single and polycrystalline deformation behavior with various heat treatments. Aluminumcopper alloys are the focus in this work and their changing stress-strain behavior is demonstrated resulting from the different hardening mechanisms brought about by the various precipitates. Extensive transmission electron microscopy investigations facilitated the interpretation of the stress-strain behavior and the work hardening characteristics. The use of both single and polycrystals proved valuable in understanding the role of anisotropy due to crystal orientation vs precipitate-induced anisotropy. The experiments show that precipitation-induced anisotropy could offset the crystal orientation anisotropy depending on the orientation. This is clearly demonstrated with similar [111] and [123] behaviors under 190 °C and 260 °C aging temperatures. Experiments on pure aluminum crystals are also provided for comparison and understanding the crystal anisotropy in the absence of precipitates. Part I of this article will focus on experiments, and part II will describe the modeling of the effect of different metastable phases in the matrix acting as barriers to dislocation motion. FOGLESONG for-merly with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801     

The microstructure and recrystallization of flow-formed oxide-dispersion-strengthened ferritic alloy: Part I. Deformation structure

Oxide-dispersion-strengthened (ODS) alloy tubes with a nominal composition of Fe-19.5 pct Cr-5.5 pct Al-0.5 pct Y₂O₃ have been cold flow formed to deformation levels between 72 and 92 pct. The deformation structure of the tubes has been studied using a variety of techniques including transmission electron microscopy (TEM). The deformation cells produced by flow forming are elongated in both the hoop and axial directions, especially at deformation levels above 80 pct. In this case, most deformation cells can be regarded as ribbons, lying roughly along the “rolling direction,” which for flow forming is a helix around the tube surface. No obvious particle alignment was found in the tubes. Although the initial grain size is submicron, transition bands composed of parallel-sided long deformation cells similar to those in deformed single-crystal specimens have been observed in the transverse section of the tubes and a macroscopic shear band has been observed in the longitudinal section of the 92 pct deformed tube.     

The effects of superimposed hydrostatic pressure on deformation and fracture: Part I. Monolithic 6061 aluminum

The effects of various levels of superimposed hydrostatic pressure on the tensile ductility and fracture micromechanisms were determined for 6061 specimens heat-treated to underaged and overaged conditions of equivalent yield strength. Superimposed pressures of 0.1, 150, and 300 MPa were selected; the ductility increased between 0.1 and 150 MPa and remained constant between 150 and 300 MPa. It is shown that the levels of pressure chosen inhibit void growth and coalescence. Void nucleation occurred at nonmetallic inclusions, and neither the ductility nor pressure response were significantly affected by the heat treatments chosen. This article is based on a presentation made in the symposium “Quasi-Brittle Fracture” presented during the TMS fall meeting, Cincinnati, OH, October 21–24, 1991, under the auspices of the TMS Mechanical Metallurgy Committee and the ASM/MSD Flow and Fracture Committee.     

Relationship between fracture toughness, fracutre path, and microstructure of 7050 aluminum alloy: Part II. Multiple micromechanisms-based fracture toughness model

A multiple micromechanisms-based model is developed to quantitatively relate the fracture toughness of partially recrystallized 7XXX aluminum alloys to their fracture surface morphology. The model is verified using the experimental data on partially recrystallized 7050 alloy reported in the companion article. It is then used to obtain a quantitative relationship between the fracture toughness and microstructural attributes. The model relates fracture toughness to microstructural parameters such as degree of recrystallization, grain size of recrystallized grains, thickness of recrystallized regions, total surface area of the constituent particles per unit volume, and microstructural anisotropy. The model predicts the changes in the fracture toughness with the specimen orientation.     

Infiltration of fiber preforms by a binary alloy: Part I. Theory

During infiltration of a fiber preform by a binary hypoeutectic alloy, solid metal can form in the composite because of cooling at the fibers or at the mold wall. Contrary to the case of an unalloyed matrix, temperature, composition, and fraction solid may vary in the composite. This results in macrosegregation and microstructural heterogeneity within the composite casting. It is shown that solid metal that forms because of cooling at the fibers grows gradually behind the infiltration front, while the local temperature increases. Metal superheat, when present, serves to progressively remelt solid metal in the composite during infiltration and increases compositional and microstructural heterogeneity within the composite. General expressions are derived to describe heat, mass, and fluid flow during the infiltration process. In the case of unidirectional adiabatic infiltration driven by a constant applied pressure, a similarity method can be used to reduce the mathematical complexity of the problem. Numerical solution of the resulting equations then allows us to predict temperature, fraction solid, and composition profiles within the composite. With the further assumption of negligible thermal conduction, the problem lends itself to an analytical solution. The analysis is performed for the case of unidirectional adiabatic infiltration under constant applied pressure of 24 vol pct δ-alumina preforms by Al-4.5 wt pct Cu. Results indicate that there is significant latitude for control of macro-segregation and microstructure within cast fiber-reinforced alloys.     

Effect of overaging and particle size on tensile deformation and fracture of particle-reinforced aluminum matrix composites

The effect of reinforcement particle size and overaging treatment on the tensile behavior and fracture morphology of a 2080/SiC/20 _p composite was investigated. Tensile behavior was profoundly influenced by particle size and matrix strength. The composite strength increased with a decrease in particle size, while overaging greatly reduced the strength of the composite, independent of particle size. Almost all particles on the fracture plane were fractured, and the amount of particle fracture in the composites was insensitive to overaging and particle size, due to the excellent bonding between SiC particles and the Al matrix. Fractography showed that void nucleation in the matrix of peak-aged composites took place primarily at very fine SiC particles, which were much smaller than the average SiC particle size. Subsequent failure took place by the tearing topography surface (TTS) mechanism. In the overaged composite, composites failed by a more conventional void nucleation and growth process, where void nucleation took place at coarsened S precipitate particles, resulting in smaller and more elongated voids.     

Forging limits for an aluminum matrix composite: Part II. Analysis

Plasticity analysis has been carried out to calculate the forging limits of a particulate-reinforced aluminum matrix composite under various forging conditions. A geometric defect(i.e., variation in cross-sectional area) that can concentrate stresses and strain and accelerate local deformation was assumed to represent all possible defects in this material. Thus, in effect, the local stress concentrations around nondeformable particles, inhomogeneous distribution of particles and grain sizes, porosity and cracked particles,etc., were assumed to be simulated adequately by such a defect factor. The analysis followed a method suggested by Marciniak and Kuczynski (MK) to determine the strain path within the defect region of the composite during multiaxial deformation. A relationship of stress, strain, and strain rate obtained from the uniaxial tension test was used for the calculation of the strains. To terminate the plasticity analysis, a rateindependent fracture criterion was used that is based on Cockcroft’s model of a constant work performed by the tensile component of stress. It was found that the calculated results predicted the experimental forging limits for 2014 Al/15 vol pct A1₂O₃ reasonably well. At 400 °C and a slow strain rate (0.015 s~’), the predicted curve was higher than the experimental result. This was probably because the fracture mechanism, and thus fracture criterion, changed with temperature. The effect of assumed defect factor on predicted forging limits was also studied. It was found that the size of the defect factor did not significantly change the forging limits at 300 °C for strain rates from 0.015 s to 0.5 s^-1; however, it did have a large effect on the forging limit at 400 °C for the lower strain rate of 0.015 s^-1. Formerly Graduate Student, the University of Michigan     

Directional dendritic solidification of a composite slurry: Part I. Dendrite morphology

A simple analytical model to describe the morphology of a growing dendrite in the presence of an inert particle has been presented. The presence of a particle changes the solute concentration gradient at the tip of a growing dendrite and this, in turn, affects the dendrite tip radius. Results of the analysis show that the dendrite tip radius decreases at a high growth velocity due to the presence of a particle, while there is no influence on the tip radius at both low and intermediate growth velocities. Lower thermal conductivity of the particle decreases the tip radius, while a higher thermal conductivity increases the radius. However, the effect of thermal conductivity on the tip radius is only significant in the cellular growth regime. The analysis shows that the presence of SiC particles in Al-Cu alloys reduces the cell to dendrite transition velocity. Results of directional solidification experiments carried out on an Al-4.5Cu-SiC composite system agree with our model.     

Short crack growth behavior in a particulate-reinforced aluminum alloy composite

Short and long crack propagation behaviors in a coarse A1₂O₃ particulate-reinforced 6061 aluminum alloy composite (Al₂O₃/6061 Al) are investigated and compared under different ranges of tensile-compressive cyclic stress. It is found that short cracks up to 400 μm in length propagate in a shear-dominant mode at maximum cyclic stress level below the fatigue limit until they are permanently trapped by the surrounding particles. The microstructure sensitivity of short crack growth in the composite decreases as the short crack length and/or applied stress range increase. The characteristics of short cracks and the mechanisms of short crack trapping by particles in the material are discussed. leave from Taiynan University of Technology This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

A Study of Scrap Heating By Burners. Part I: Experiments

A furnace, approximately 1 m³ in capacity, was built to measure the efficiency of heat transfer from a propane-oxygen burner to different types of industrial steel scrap. Supplementary measurements were undertaken to measure the scrap density, size, and void fraction to insure complete combustion of the propane and to measure the extent of scrap oxidation. Local values of temperature in the scrap bed were measured along with the exit temperature of the off gas. The off gas temperature was a direct measurement of the overall efficiency of heating. It was found that smaller and denser scrap was heated more effectively. Configurations that increased the path length of the gas also increased the efficiency of heating. In Part II, a mathematical model is developed to compare with the measured temperatures.     

Deformation behavior of a Ni3Al(B,Zr) alloy during cold rolling: Part I. changes in order and structure

A hypostoichiometric Ni₃Al(B,Zr) alloy was homogenized and cold rolled by amounts ranging from 25 to 73 pct. The alloy consisted of two phases—a partially ordered γ′ phase (L1₂) and a Ni-rich fcc solid solution (γ). On deforming the alloy by rolling at room temperature, the order parameter showed a gradual change. In fact, between 35 and 45 pct deformation, the order characteristic of the L1₂ structure changed into that of a DO₂₂ structure. The possibility of transition from L1₂ to DO₂₂ structure is also corroborated from strain parameter, microhardness, and detailed x-ray diffraction (XRD) measurements. This structural transformation is accompanied by a change in the deformation mode (from slip to twinning), as is evident from the relevant microstructures.     

Investigation of Freeze-Linings in Copper-Containing Slag Systems: Part I. Preliminary Experiments

Slag freeze-linings are increasingly used in industrial pyrometallurgical processes to insure that furnace integrity is maintained in aggressive high-temperature environments. Most previous studies of freeze-linings have analyzed the formation of slag deposits based solely on heat-transfer models. The focus of the present research is to determine the impact of slag chemistry and local process conditions on the microstructures, thickness, stability, and heat-transfer characteristics of the frozen deposit at steady-state conditions. The formation of the freeze-linings is studied under controlled laboratory conditions using an air-cooled “cold-finger” technique for Cu-Fe-Si-Al-O slag at equilibrium with metallic copper relevant to the industrial copper smelting processes. The phase assemblages and microstructures of the deposits formed in the cold-finger experiments differ significantly from those expected from phase equilibrium considerations. The freeze-lining deposits have been found, in general, to consist of several layers. Starting from the cold finger, these layers consist of glass; glass with microcrystalline precipitates; closed crystalline layer; and open crystalline layer. Even at steady-state conditions, there was no primary phase sealing layer of delafossite [Cu₂O · (Al, Fe)₂O₃] present at the deposit/liquid interface—these observations differ markedly from those expected from phase equilibrium considerations. The findings have significant practical implications, and potential for the improved design and operation of industrial metallurgical furnaces.